Greetings:
Since the prepatch several mounts (especially horse models that are capable of flying) have bugged ground riding animations in comparison to their “pure” ground mount versions.
https://youtu.be/ycRPPS3Y3Z0
This short vid contains some of the horses and a short comparison between those.
tldr:
.) The “new” horse models have over the board a failing riding animation, no matter if they can fly or not – BUT the flying animation is fine.
.) The “old” models have the usual riding animation, BUT the horses with flying capabilities (Headless Horseman Mount) instead have a failing ground riding animation while the flying animation is just fine.